Stream-oriented interconnect for networked computer storage

ABSTRACT

An apparatus and method for connecting a plurality of computing devices, e.g. web servers, database servers, etc., to a plurality of storage devices, such as disks, disk arrays, tapes, etc., by using a stream-oriented (circuit oriented) switch that has high throughput, but that requires non-negligible time for reconfiguration is disclosed. An example of such stream-oriented switch is an optical switch. The preferred embodiment comprises a plurality of communication ports for connection to servers, and plurality of ports for connection to storage devices. The system decodes the requests from the computing devices and uses this information to create circuits, e.g. optical paths in embodiments where the stream-oriented switch is an optical switch, through the stream-oriented switch. The system uses these circuits to route traffic between the computing devices and the storage devices. Buffering data and control in the device memory is used to improve overall throughput and reduce the time spent on reconfigurations.

This application claims the benefit of Provisional Application No. 60/292,106, filed May 17, 2001.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to the storage of information. More particularly, the invention relates to a stream-oriented interconnect for networked computer storage.

2. Description of the Prior Art

The properties of typical traffic between computing devices and storage devices are fundamentally different from those of the typical flows between computing devices. For example, the latency between the time a request is issued for a disk read and/or write operation and the time the operation is performed by the disk can run into the multiple milliseconds due to factors such as disk seek time and disk rotational delays. As opposed to typical Internet traffic, which tends to have short bursts of traffic to different servers, the traffic in a storage network tends to have significantly longer packets and, in many cases, is stream (circuit) oriented and predictable. In addition, while in general it is very important to minimize the latency in computer networks, the latency of write-to-disk operations is relatively unimportant in many cases.

Known stream-oriented switches, e.g. optical interconnects based on micro-mirrors, electro-optic, thermo-optic, acousto-optic, bubbles, etc., are capable of very high throughput but have relatively long switching/reconfiguration times. Conversely, conventional electronic packet switches have lower maximum throughput than optical switches, but have significantly faster switching times. Switching time, as used herein, refers to the time that elapses between a command to create a connection between ports and the time when the data can start flowing through the system.

It would be advantageous to provide an apparatus and method for connecting a plurality of computing devices, e.g. web servers, database servers, etc., to a plurality of storage devices, such as disks, disk arrays, tapes, etc., by using a stream-oriented (circuit oriented) switch that has high throughput, but that requires non-negligible time for reconfiguration.

SUMMARY OF THE INVENTION

The presently preferred embodiment of the invention comprises an apparatus and method for connecting a plurality of computing devices, e.g. web servers, database servers, etc., to a plurality of storage devices, such as disks, disk arrays, tapes, etc., by using a stream-oriented (circuit oriented) switch that has high throughput, but that requires non-negligible time for reconfiguration. An example of such stream-oriented switch is an optical switch.

The preferred embodiment comprises a plurality of communication ports for connection to servers, and a plurality of ports for connection to storage devices. The system decodes the requests from the computing devices and uses this information to create circuits, e.g. optical paths in embodiments where the stream-oriented switch is an optical switch, through the stream-oriented switch. The system uses these circuits to route traffic between the computing devices and the storage devices. Buffering of data and control in the device memory is used to improve overall throughput and reduce the time spent on reconfigurations.

An alternative embodiment of the system uses two interconnects, in which all devices are connected to one or both interconnects. One of the interconnects is the stream-oriented switch described above. The second interconnect is a conventional packet switch which can switch traffic on a packet-by-packet basis, e.g. electronic packet switch. In contrast to the stream-oriented switch, the packet switch has much smaller overall throughput, but requires much less time for reconfiguration. The stream-oriented switch is used for switching relatively large data streams, e.g. reading a large file, while the packet switch is used for control and the rest of the data traffic.

A further embodiment of the invention comprises a system that uses a statistical prediction algorithm, e.g. HMM, Hidden Markov Model, to predict traffic flows. It then uses a statistical decision algorithm, e.g. MDP, Markov Decision Process, to decide on how to reconfigure the stream-oriented switch at every moment in time. The system attempts to optimize certain customer selectable measures, such as throughput, latency, etc., by configuring the stream-oriented switch such that the selected measures are optimized for the predicted traffic, while routing the remainder of the traffic, i.e. the predicted traffic that could not be accommodated by the high-throughput interconnect, as well as the unpredicted traffic, through the packet switch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block schematic diagram that shows device for switching storage-related data using an optical switch according to the invention;

FIG. 2 is a block schematic diagram that shows section of a dual fabric interconnect according to the invention;

FIG. 3 is a block schematic diagram that shows a dual fabric section after an optical switch is switched to disk B according to the invention; and

FIG. 4 is a block schematic diagram that shows a generalized overview of a dual fabric architecture according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Networked Storage Using an Optical Switch

As shown in FIG. 1, the herein disclosed invention, a stream-oriented, optical switch 12 (also referred to herein as an optical switch), connects a plurality of computing devices 14 to a plurality of storage devices 16. The presently preferred embodiment of the invention comprises an optical switching device 11, a switch control unit 13, a scheduling and load balancing unit (called hereinafter SLBU) 15, and a system controller 17.

The system controller 17 is responsible for the overall control of the entire system. It communicates with the computing and storage devices, gathering their request and monitoring status information. This information is distributed to the SLBU, which determines a performance enhancing packet schedule and routing as well as switch configuration. The system controller then distributes the packet schedule, routing information, and stream-oriented switch configuration to the relevant parts of the system.

The SLBU is responsible for distributing the communication load between the various available links between the computing and storage devices. It is also responsible for ordering the packet flows through the system. It attempts to minimize frequency of stream-oriented switch reconfigurations. Such reconfigurations generally take time, thereby increasing latency, and decreasing throughput. SLBU is also concerned with providing quality-of-service to the computing and storage devices

The packet scheduling unit 19 sends a transmission schedule to the computing and storage devices as well as to the system controller. Based on the information provided by the SLBU, the system controller determines when a switch configuration change is required, and sends a command to the switch control unit to modify the optical switch configuration. Once the optical switch has been reconfigured, the switch control unit informs the rest of the system of the new configuration, and the computing and storage devices commence using the circuits that are available through this new configuration for communicating.

A Multiple Fabric Switch for Connecting Computing and Storage Devices

In this embodiment of the invention, all devices are connected to one, two, or more interconnects, e.g. an electronic packet switch and an optical switch. The packet switch is used for some of the control and data traffic, while the optical switch is used for the remainder of the traffic. The multiple fabrics are controlled by the system control unit, which receives and issues control and status information via the packet switch and which is responsible for configuring the optical switch.

The system decodes the traffic and splits it in two. The control part of the traffic 20 is sent through the packet switch, while the rest of the traffic 22 is routed through the optical switch. A subsection of a system based on this invention with two interconnects is shown in FIG. 2, in which one computing device 21 is connected to two storage devices 23, 25 both through a packet switch, and a circuit created by an optical switch. In the system state illustrated in FIG. 2, the circuit (in the example, an optical path 22, 27) connects the computing device and Disk A.

If the traffic flow changes and it becomes advantageous to create a circuit between the computing device and Disk B, the configuration of the optical switch is changed to the configuration that is shown in FIG. 3, where the optical switch implements a circuit (in this example, an optical path 22, 31) that connects the computing device and Disk B. While the system is in this configuration, the packets that need to be transmitted between the computing device and Disk A are flowing through the packet switch, illustrated by the arrows 20.

While two disks are shown in this example, those skilled in the art will appreciate that any number of storage devices may be used in connection with the herein disclosed invention. Similarly, those skilled in the art will be able to use an optical switch that supports multiple concurrent circuits as opposed to a single circuit shown in FIGS. 2 and 3.

This embodiment of the invention is controlled by a system controller that is responsible for the overall control of the system. It communicates with the computing and storage devices, gathering their request and status information, and distributing this information to the SLBU.

The SLBU is responsible for distributing the communication load between the various available links through the fabrics connecting the computing and storage devices. It recommends to the system controller unit a configuration of the optical switch and the packet switch that divides the traffic between the two fabrics to optimize certain customer selectable criteria such as throughput and delay.

The SLBU is also responsible for ordering the packet flows through the system. It attempts to balance the frequency of optical switch reconfigurations (which, generally, take time, thereby increasing latency and decreasing throughput), with optimizing the customer-desired system attributes, e.g. total throughput. The SLBU communicates with the computing and storage devices for the most part via the packet switch. The SLBU sends a transmission schedule to the computing and storage devices, as well as to the system controller. (Alternatively, as shown in FIG. 4, SLBU can communicate with splitters/combiners making the switch look like a single-fabric switch from the point of view of the storage and the computer devices.) Based on the information provided by the SLBU, the system controller determines when an optical switch configuration change is required, and sends a command to the switch control unit to modify the optical switch configuration. This is shown in FIG. 2 by the arrow marked “mirror control.”

Additionally, because in many cases a seek operation by the target disk device is required, such a command can be sent to the target via the packet switch in parallel with the reconfiguration of the optical switch. The arrow marked “disk control” illustrates this. Once the optical switch has been reconfigured, as illustrated in FIG. 3, the switch control unit informs the rest of the system of the new configuration, and the computing and storage devices commence communicating via this new configuration.

An overview of a presently preferred embodiment of the system is shown in FIG. 4. Packets sent from computing devices 14 are decoded by respective splitters/combiners 50 (one such device per port) and some control/status information is extracted and forwarded to the load balancer SLBU 15. The SLBU controls the state of the optical switch 11 (optical interconnect in this example) and instructs the load balancers/splitters which of the packets/streams should be routed through the optical switch, i.e. optical interconnect. The rest of the packets are routed through the packet switch 51 (in this example, the electrical interconnect). Splitters/combiners can be either part of the dual-fabric switch or embedded into computing and storage devices.

An example of a transaction in such a system is a read operation by a computing device. In such a case, the computing device issues a request for the data from a storage device. The read request is decoded by the appropriate splitter/combiner. It is immediately forwarded through the packet switch, e.g. electrical interconnect, to the appropriate storage device. At the same time, the information describing the request, e.g. number of bytes to be transmitted and source/destination, is forwarded to the load balancer. While the storage device performs the seek operation, the load balancer instructs the optical switch, e.g. the optical interconnect, to prepare a data path between the appropriate ports. When the path is prepared, it instructs the splitter/combiner responsible for the storage device port to route the data from the storage device through the optical switch. It also instructs the appropriate splitter/combiner on the server side to be ready for data that will shortly arrive from the optical switch. The splitter/combiner receives this data and forwards it to the computing device.

A similar procedure is executed for computing device writes to the storage devices. In general, the SLBU monitors the data streams that are passing through the system. It also monitors the overall throughput and the data transfers that are in progress, redirecting some of the transfers to pass through the optical switch, e.g. optical interconnect. The rest of the data is forwarded through the packet switch, e.g. electrical interconnect. Observe that only meta-data, such as request descriptions, are passed through the SLBU. The actual data flows through the two interconnects.

In some cases, it is advantageous to use buffering to aggregate multiple data write requests, as well as to use data caching and predictive data read-ahead to improve performance by increasing the amount of data that can be transferred through the optical switch between reconfigurations. Using two network adapters in each server can eliminate the need for splitters/combiners on the server side. In this case, one of these adapters is connected to the packet switch, e.g. electrical interconnect, and the load balancer, e.g. using Gigabit Ethernet, while the other adapter is connected to the optical switch, e.g. optical interconnect, using optical interface cards such as Fibre Channel. In this case, it is possible to use a server-side driver, e.g. a software package, that collects global data about the requests and sends it to the load balancer, e.g. through Gigabit Ethernet.

Reconfiguration of optical switches can often require a non-negligible amount of time. To improve performance, one embodiment uses more than one optical switch. In this case, the SLBU instructs the splitters/combiners to forward data through interconnects that are already reconfigured, while reconfiguring other interconnects at the same time. This causes reconfiguration intervals of one optical switch, e.g. in the case of an optical interconnect, the interval of time when the optical interconnect is being reconfigured and hence is unavailable for forwarding packets, to overlap the intervals of time when the other interconnect, or interconnects, are already reconfigured and hence can forward the packets. To improve performance, the system supports the connection of a single server to several ports, e.g. by using several network adapters, and the connection of a single storage device to several ports. To reduce cost, the system supports the connection of several servers and/or storage devices to a single port.

A Prediction System Based on an Adaptive Statistical System Model

Computing device requests can be predicted with some degree of accuracy. For example, if a computing device has read the first seven sectors of a 10-sector file in quick succession, it is it quite likely that the computing device will soon request the remaining three sectors. This embodiment of the invention provides a performance enhancement system in the form of a transaction prediction system that predicts future behavior of the computing and storage devices based on prior transactions and known behavior patterns, e.g. the read example discussed above. The system builds and continually adapts a statistical model of the system, for example using a Hidden Markov Model, HMM. The system improves from time to time the statistical model and uses it to enhance the performance of the herein disclosed optical switch by making predictions about the future transactions of the system.

For example, consider a system as shown in FIG. 2, where the computer has read 300 sectors from file X, which contains 350 sectors and resides on Disk A, and then issues a read request for two sectors from file Y, that reside on Disk B. Without a prediction mechanism, the system may choose to reconfigure the optical switch to create a path between the computing device and Disk B. However, it is quite likely that the computing device would soon request the remaining 50 sectors of file X. Therefore, in most cases, better overall throughput and lower latency would be achieved by routing the sectors from file Y through the packet switch, and leaving the optical switch circuit established between storage the computing device and Disk A.

The decision of when to reconfigure the optical switch and how to reconfigure it can be made using, for example, a Markov Decision Process, MDP. In this case, the maintained statistical model is used to predict the expected benefit, e.g. performance improvement, of each possible reconfiguration (or no reconfiguration) at every step and to choose the reconfiguration (or a sequence of reconfigurations) that achieves significant improvement in the sense of mathematical expectation. When preparing the reconfiguration at each step, the system can also take into account possible errors in the predictions of the statistical model, as well as the statistical confidence in the predictions. Performance can also be enhanced by using the adaptive statistical model to predict reads and writes, therefore pre-positioning the disk heads, thereby decreasing latency, and enhancing overall system throughput. In the case where this prediction is implemented in the context of the optical switch, and the actual system behavior differs from the predicted behavior, the packets that have not been predicted can be either temporary buffered in the system and/or routed though the packet switch, thus not incurring the latency penalty that may be imposed by a required reconfiguration of the optical switch.

Dynamically Rearranging Data Stored on the Storage Devices

Using statistical analysis tools such as those mentioned above, another embodiment of the invention detects instances where the data are distributed among the storage devices in a less than optimal way. For example, consider the case where a large video file that is usually accessed sequentially by a single computing device at a time is split among several storage devices connected to several different ports on the system. In this case, it might be advantageous to relocate this file to a single storage device or to plurality of storage devices attached to a single port. Such relocation reduces the number of necessary reconfigurations of the optical switch that are needed for optimum access to the file.

It is important to note that there is a tradeoff in such relocation. While the file is split among several storage devices, it is theoretically possible to access it concurrently from several different computing devices, increasing the overall throughput. On the other hand, this (split) configuration might require extra reconfigurations of the optical switch. Based on the adaptive statistical access model mentioned above, the SLBU uses a decision policy, e.g. based on Markov Decision Process, to analyze these advantages and disadvantages and decides whether to relocate the file and where to relocate it. Note that the use of a file here is an example. A similar approach can be used for relocating sectors, blocks, or regions of data. The approach also can be applied if the storage device uses virtualization to hide its actual physical storage architecture/configuration.

Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the claims included below. 

1. An apparatus for interconnecting any of plurality of computing devices and a plurality of storage devices, comprising: a stream-oriented switch having a high throughput, but that requires non-negligible time for reconfiguration; said stream-oriented switch further comprising: a plurality of communication ports for connection to any of said computing devices and a plurality of ports for connection to said storage devices; means for decoding requests from any of said computing devices and/or storage devices and for using information from said decoded requests to create circuits through said stream-oriented switch; means for using said circuits to route traffic between any of said computing devices and said storage devices; and means for buffering data and control information to improve overall throughput and reduce time spent on reconfiguration.
 2. The apparatus of claim 1, wherein said stream-oriented switch is an optical switch.
 3. The apparatus of claim 1, further comprising: at least two interconnects; wherein all devices are connected to at least one of said interconnects; wherein one of said interconnects comprises said stream-oriented switch; wherein at least a second interconnect comprises a conventional packet switch which can switch traffic on a packet-by-packet basis; and wherein said stream-oriented switch is used for switching relatively large data streams, while said packet switch is used for control and the rest of data traffic.
 4. The apparatus of claim 3, further comprising any of: a statistical prediction algorithm for predicting traffic flows; and a statistical decision algorithm for deciding on how to reconfigure said stream-oriented switch in real time.
 5. The apparatus of claim 4, wherein certain selected measures are optimized by configuring said stream-oriented switch for routing said predicted traffic flow, and wherein a remainder of predicted traffic that could not be accommodated by said stream-oriented switch, as well as unpredicted traffic, through said packet switch.
 6. The apparatus of claim 4, wherein said statistical prediction algorithm comprises a Hidden Markov Model; and wherein said statistical decision algorithm comprises a Markov Decision Process.
 7. An apparatus for interconnecting any of plurality of computing devices and a plurality of storage devices, comprising: an optical switch; a switch control unit for modifying said optical switch configuration, wherein once said optical switch is reconfigured, said switch control unit generates information indicating said reconfiguration, and wherein any of said computing devices and said storage devices commence using circuits that are available through said reconfiguration for communicating; a scheduling and load balancing unit for determining a performance enhancing packet schedule and routing, for generating information used in determining switch configuration, for distributing a communication load among various available links between any of said computing devices and said storage devices, and for ordering packet flows; and a system controller for communicating with said computing devices and said storage devices, for gathering requests, for monitoring status information, for distributing a packet schedule, for routing information, and for effecting stream-oriented switch configuration.
 8. The apparatus of claim 7, further comprising: a packet scheduling unit for sending a transmission schedule to said computing devices and said storage devices, as well as to said system controller.
 9. The apparatus of claim 7, said optical switch comprising a multiple fabric switch for connecting said computing devices and said storage devices.
 10. The apparatus of claim 9, wherein all said devices are connected to at least one of an electronic packet switch and an optical switch; wherein said packet switch is used for control and a portion of data traffic; wherein said optical switch is used for a remainder of said data traffic; and wherein said multiple fabrics are controlled by said system control unit which receives and issues control and status information via said packet switch, and which is responsible for configuring said optical switch.
 11. The apparatus of claim 10, wherein said scheduling and load balancing unit distributes a communication load between available links through said fabrics which connect said computing devices and said storage devices by recommending to said system controller unit a configuration of said optical switch and said packet switch that divides traffic between said fabrics based upon selected criteria.
 12. The apparatus of claim 11, wherein said selected criteria comprise any of throughput and delay.
 13. The apparatus of claim 7, further comprising any of: a buffer for aggregating multiple data write requests; and a data caching and predictive data read-ahead mechanism; wherein amount of data that can be transferred through said optical switch between reconfigurations is increased.
 14. The apparatus of claim 7, further comprising: a transaction prediction system that predicts future behavior of said computing devices and said storage devices based on prior transactions and known behavior patterns.
 15. The apparatus of claim 14, wherein said transaction prediction system builds and continually adapts a statistical model, which optionally comprises a Hidden Markov Model; wherein said transaction prediction system improves said statistical model from time to time and uses said statistical model to enhance performance of said optical switch by making predictions about future transactions.
 16. The apparatus of claim 14, wherein a decision of when and how to reconfigure said optical switch is made using a Markov Decision Process.
 17. The apparatus of claim 14, wherein a maintained statistical model is used to predict an expected benefit of each possible reconfiguration decision at every step, and to choose a preferred reconfiguration or reconfiguration sequence.
 18. The apparatus of claim 7, further comprising: means for detecting instances where data are distributed among said storage devices in a less than optimal way; and means for dynamically rearranging data stored on said storage devices.
 19. The apparatus of claim 18, further comprising: an adaptive statistical access model for analyzing data redistribution advantages and disadvantages and for deciding whether to relocate said data and, if so, where to relocate said data.
 20. A method for interconnecting any of plurality of computing devices and a plurality of storage devices, comprising the steps of: providing a stream-oriented switch having a high throughput, but that requires non-negligible time for reconfiguration; providing a plurality of communication ports for connection to any of said computing devices and said storage devices; decoding requests from any of said computing devices and said storage devices, and for using information from said decoded requests to create circuits through said stream-oriented switch; using said circuits to route traffic between any of said computing devices and said storage devices; and buffering data and control information to improve overall throughput and reduce time spent on reconfiguration.
 21. The method of claim 20, wherein said stream-oriented switch is an optical switch.
 22. The method of claim 20, further comprising the step of: providing at least two interconnects; wherein all devices are connected to at least one of said interconnects; wherein one of said interconnects comprises said stream-oriented switch; wherein at least a second interconnect comprises a conventional packet switch which can switch traffic on a packet-by-packet basis; and wherein said stream-oriented switch is used for switching relatively large data streams, while said packet switch is used for control and the rest of data traffic.
 23. The method of claim 22, further comprising the step of providing any of: a statistical prediction algorithm for predicting traffic flows; and a statistical decision algorithm for deciding on how to reconfigure said stream-oriented switch in real time.
 24. The method of claim 23, wherein certain selected measures are optimized by configuring said stream-oriented switch for routing said predicted traffic flow, and wherein a remainder of predicted traffic that could not be accommodated by said stream-oriented switch, as well as unpredicted traffic, through said packet switch.
 25. The method of claim 23, wherein said statistical prediction algorithm comprises a Hidden Markov Model; and wherein said statistical decision algorithm comprises a Markov Decision Process.
 26. A method for interconnecting any of plurality of computing devices and a plurality of storage devices, comprising the steps of; providing an optical switch; providing a switch control unit for modifying said optical switch configuration, wherein once said optical switch is reconfigured, said switch control unit generates information indicating said reconfiguration, and wherein said any of computing devices and said storage devices commence using circuits that are available through said reconfiguration for communicating; providing a scheduling and load balancing unit for determining a performance enhancing packet schedule and routing, for generating information used in determining switch configuration, for distributing a communication load among various available links between any of said computing devices and said storage devices, and for ordering packet flows; and providing a system controller for communicating with any of said computing devices and said storage devices, for gathering requests, for monitoring status information, for distributing a packet schedule, for routing information, and for effecting stream-oriented switch configuration.
 27. The method of claim 26, further comprising the step of: providing a packet scheduling unit for sending a transmission schedule to said computing devices and said storage devices, as well as to said system controller.
 28. The method of claim 26, said optical switch comprising a multiple fabric switch for connecting said computing devices and said storage devices.
 29. The method of claim 28, wherein all said devices are connected to at least one of an electronic packet switch and an optical switch; wherein said packet switch is used for control and a portion of data traffic; wherein said optical switch is used for a remainder of said data traffic; and wherein said multiple fabrics are controlled by said system control unit which receives and issues control and status information via said packet switch, and which is responsible for configuring said optical switch.
 30. The method of claim 29, wherein said scheduling and load balancing unit distributes a communication load between available links through said fabrics which connect said computing devices and said storage devices by recommending to said system controller unit a configuration of said optical switch and said packet switch that divides traffic between said fabrics based upon selected criteria.
 31. The method of claim 30, wherein said selected criteria comprise any of throughput and delay.
 32. The method of claim 26, further comprising the step of providing any of: a buffer for aggregating multiple data write requests; and a data caching and predictive data read-ahead mechanism; wherein amount of data that can be transferred through said optical switch between reconfigurations is increased.
 33. The method of claim 26, further comprising the step of: providing a transaction prediction system that predicts future behavior of said computing devices and said storage devices based on prior transactions and known behavior patterns.
 34. The method of claim 33, wherein said transaction prediction system builds and continually adapts a statistical model, which optionally comprises a Hidden Markov Model; wherein said transaction prediction system improves said statistical model from time to time and uses said statistical model to enhance performance of said optical switch by making predictions about future transactions.
 35. The method of claim 33, wherein a decision of when and how to reconfigure said optical switch is made using a Markov Decision Process.
 36. The method of claim 33, wherein a maintained statistical model is used to predict an expected benefit of each possible reconfiguration decision at every step, and to choose a preferred reconfiguration or reconfiguration sequence.
 37. The method of claim 26, further comprising the step of: detecting instances where data are distributed among said storage devices in a less than optimal way; and means for dynamically rearranging data stored on said storage devices.
 38. The method of claim 37, further comprising the step of: providing an adaptive statistical access model for analyzing data redistribution advantages and disadvantages and for deciding whether to relocate said data and, if so, where to relocate said data. 